1. Field of the Invention
The present invention relates to a method for setting a program profile in the control of the injection speed of injection molding machines.
2. Related Art Statement
In injection molding machines, the control of the moving speed of the injecting plunger (screw) in a forward direction is usually conducted in accordance with a program profile, which is preliminarily set up in a computer.
Conventionally the profile is generally set up in the computer in such a manner that a resin material filling stroke of the injection plunger is divided into a plurality of phases, and the moving speed of the plunger in a forward direction is directly designated at every phase to obtain in a multi-step rectangular-shaped profile.
FIG. 1 is a graph showing a conventional program profile in the control of the moving speed of an injection plunger. As shown in FIG. 1, the filling stroke of the injection plunger is divided into five phases, and the moving speed of the plunger V1 to V5 is directly designated at every phase to obtain the multi-step rectangular-shaped profile.
As shown in FIG. 1, the multi-step rectangular shaped profile would result in sudden changes in the moving speed of the injection plunger in a forward direction. However, in an actual molding process, the cross-section of the course of the melted resin filling the mold die does not suddenly change. Thus a conventional multi-step rectangular-shaped profile does not conform to an actual movement of the melt resin in the mold die.
To obtain a profile which corresponds to the actual shape of the course of the melted resin in the cavity, it is necessary to increase the number of steps of the rectangular-shaped profile. Accordingly, the number of phases to divide the material filling stroke into should also be increased.
Furthermore, as shown in FIG. 1 by broken lines, the actual moving speed of the injection plunger in a forward direction has some delays on the leading and trailing edges, that is, the moving speed of the injection plunger does not immediately switch from one speed level to the next as required by the program profile. Therefore, as apparent from FIG. 1, the actual moving speed of the injection plunger does not change according to the multi-step rectangular-shaped profile set up in the computer.
In general, the program profile controlling the injection speed is set speed so as to complete the filling up of the melt resin within as short a period as possible without disturbing the flowing condition of the melted resin in the cavity. Therefore, the program profile should be set up whereby the moving speed of the plunger is changed so as to comply with the actual shape of the course of the melted resin in the cavity. Furthermore, in order to set up such a profile, a basic knowledge of molding principle is necessary to control the moving speed of plunger when the melted resin starts to be injected from the gate or when the mold die has been filled up with resin.
Therefore, the actual profile of the moving speed of the injection plunger (hereinafter referred to as "a prosecuting profile") does not correctly correspond to the program profile in the control of the injection speed (hereinafter referred to as "a set-up profile"), so that the prosecuting profile and the set-up profile are not the same when superimposed on each other when a closed loop control is conducted. In order to solve this drawback, a delay control is applied, in which time constants are generated on the leading and trailing edges of the moving speed of the plunger. However, such delayed leading and trailing edges are not reflected in the set-up profile.
For these reasons, when a closed loop control is conducted, the prosecution profile deviates from the set-up profile even if the control is conditioned normally; such a deviation causes difficulty in the control of the moving speed of the plunger.
On the other hand, another method for setting a program profile is suggested where the profile corresponds to the variation of the shape of the course of the melt resin in the mold cavity. According to the method, the profile is formed by broken lines being connected together, however, it is difficult to set parameters unless the shape of the course of the melt resin in the mold cavity is well known, and it is required to have many points to be set up in order to obtain the profile which corresponds to the shape of the flowing path of the melted resin. Therefore, according to the method, it would be difficult to realize an idea that the required profile could be obtained with a small amount of inputting data. Further, the know-how for setting the parameters is much more difficult than that for setting a multi-step rectangular-shaped profile.